Project Summary Patients with obstructive sleep apnea (OSA) are at increased risk for postoperative complications. Recently, OSA was identified as an independent risk factor for postoperative delirium, a complication that affects between 10% and 70% of older patients after various major surgeries. Delirium is associated with increased morbidity, mortality, and health care costs. The possible link between OSA and delirium represents a serious and pressing public health concern and an important opportunity for improvement in health care, since about 25% of adult surgical patients have OSA. Furthermore, the majority of these patients are undiagnosed and untreated with the gold standard of continuous positive airway pressure (CPAP) therapy. Preoperative screening for OSA is routine at our institution, and identifies high-risk patients; however, it is unknown whether instituting CPAP therapy before surgery can prevent delirium and other postoperative complications. Our long- term goal is to optimize the diagnosis and treatment of OSA in the perioperative period with the objective of decreasing postoperative complications such as delirium. We propose an observational study to provide further evidence that OSA is linked to postoperative delirium, and to investigate whether preoperative CPAP therapy might decrease postoperative delirium and its negative consequences. Several characteristic features of OSA ? hypoxia, chronic inflammation, and sleep deprivation - are likely mediators of delirium and are improved by CPAP therapy. Thus, our central hypothesis is that in patients with OSA, perioperative CPAP therapy decreases the incidence of postoperative delirium and its sequelae. To characterize associations among OSA, CPAP adherence, postoperative delirium and postoperative quality of life, we propose three aims: 1) To compare the incidence of postoperative delirium in surgical patient groups based on OSA diagnosis and treatment with CPAP; 2) To assess whether untreated OSA in the preoperative period is independently associated with increased risk for postoperative delirium; and 3) To explore whether untreated OSA in the preoperative period is independently associated with decreased postoperative quality of life. If we find that CPAP therapy is associated with better outcomes, this would inform two priorities. First, it would reinforce the importance of CPAP therapy adherence in the perioperative period for patients carrying a known OSA diagnosis. Second, it would provide a strong impetus to study prospectively whether, for patients with undiagnosed OSA, definitive diagnosis and optimal treatment of OSA preoperatively with CPAP therapy would prevent delirium and improve postoperative outcomes.